1. Field of the Invention
This invention relates to an active vibration isolator used in a semiconductor exposure apparatus, which burns a circuit pattern on a reticle into a wafer, an apparatus for manufacturing a liquid crystal substrate or an electron microscope. More specifically, the invention relates to an active vibration isolator capable of suppressing external vibration transmitted to a vibration isolation platform and of positively canceling out vibration produced by a precision instrument per se, such as a stage, mounted on a vibration isolation platform, as well as to an exposure apparatus, exposure method and device manufacturing method that employ this active vibration isolator.
2. Description of the Related Art
In an electron microscope which uses an electron beam or in a semiconductor exposure apparatus typified by a stepper or scanner, an XY stage is mounted on a vibration isolator. The vibration isolator functions to attenuate vibration by vibration absorption means such as an air spring, coil spring or vibration isolating rubber. However, a problem in such a passive vibration isolator having these vibration absorption means is that vibration produced by the XY stage itself, which is mounted on the isolator, cannot be attenuated effectively even though vibration transmitted from the floor can be attenuated to some extent. In other words, a reaction force produced by high-speed movement of the XY stage per se causes the vibration isolation platform to vibrate. This vibration causes a marked decline in the positioning stability of the XY stage. Another problem with the passive vibration isolator is that there is a performance tradeoff between isolation of vibration transmitted from the floor and suppression of vibration produced by the high-speed movement of the XY stage per se. In order to solve these problems, there has been a tendency in recent years to use active vibration isolators.
An active vibration isolator is equipped with a vibration control loop which detects and feeds back vibration of the vibration isolation platform, and a position control loop for orienting the vibration isolation platform at a predetermined location. The vibration control loop is the major feature of the active vibration isolator that distinguishes it from the passive vibration isolator. The active vibration isolator makes it possible to realize a sky-hook damper (electrical viscous damping) and a sky-hook spring (electrical spring). By way of example, the specification of Japanese Patent Application Laid-Open No. 6-159433 ("Active Vibration Isolation Method and Vibration Isolation Apparatus") describes a type of feedback for obtaining the effects of both a sky-hook damper and a sky-hook spring. This specification discloses a vibration isolator in which an air spring is adopted as an actuator and expresses a nominal model of the vibration isolation platform by the following equation: ##EQU1##
where K.sub.v represents the gain of a servo valve, T.sub.v the time constant of a pneumatic system which includes the servo valve and an air spring, M the mass of the vibration isolation platform, D the viscous damping coefficient of the air spring, and K the spring constant of the air spring.
The transfer function of the first-order time lag in the first factor on the right side of Equation (1) expresses the driving characteristic of the air spring, and the transfer function of the second-order time lag in the second factor on the right side of the equation expresses the transfer function of the mechanical system. The sky-hook damper effect is obtained by acceleration feedback and the sky-hook spring effect is obtained by velocity feedback. However, the specification of the above-mentioned laid-open application does not describe why the effects of the sky-hook damper and sky-hook spring are realized. The reason why the effects of the sky-hook damper and sky-hook spring are obtained resides in the fact that T.sub.v in Equation (1) actually is chosen to be sufficiently large, so that the transfer function of the first-order time lag of the first factor is substantially an integration characteristic. In other words, owing to acceleration feedback, a signal indicative of acceleration undergoes first-order integration in accordance with the integration characteristic of the actuator and becomes a dimension of velocity, and the manipulated variable with respect to the mechanical system acts as damping (viscosity). Similarly, when velocity feedback is performed, a dimension of position is obtained by the integration characteristic of the actuator and the manipulated variable with respect to the mechanical system acts as a spring. Accordingly, the sky-hook damper effect is obtained by acceleration feedback and the sky-hook spring effect is obtained by velocity feedback.
Most of the conventional active vibration isolators apply the manipulated variable to the support mechanism of the vibration isolation platform as damping by feeding back the output of the acceleration sensor, which is the typical vibration measurement means. That is to say, only the sky-hook damper effect is utilized. In order to maximize the performance of an active vibration isolator, the sky-hook spring effect should be utilized and not just the sky-hook damper effect. Active vibration isolators equipped with a sky-hook spring along with a sky-hook damper exclude active vibration isolators that use a piezoelectric element as the actuator and are considered to be non-existent at least among active vibration isolators that use an air spring as the actuator.
The sole example is disclosed in the specification of Japanese Patent No. 2673321 "Method and Circuit for Supporting Horizontal Position and Isolating Horizontal Vibration of Vibration Isolation Platform", which describes the construction of a control apparatus for applying PID compensation to the output of an acceleration sensor and driving a servo valve that regulates the inflow and output of air, which is the working fluid, to and from an air spring, where PID represents "proportional plus integral plus derivative". Though the specification does not use the terms sky-hook damper or sky-hook spring, in actuality the sky-hook damper effect is achieved by feeding back acceleration at a proportional gain P, and the sky-hook spring effect is achieved by feeding back an acceleration signal via an integrating gain I. It should be noted that if acceleration is fed back via a differentiating gain D, mass is increased.
When the output of the acceleration sensor is fed back via the integrating gain I, however, the full integration characteristic actually cannot be brought to bear in order to avoid drift. More specifically, since the integrator is one which involves low-cut processing, a significant sky-hook spring effect cannot be manifested and, in actual practice, only the sky-hook damper effect is used. That is, though the above-mentioned specification discloses a control apparatus which feeds back the output of an acceleration sensor via a PID compensator, meaningful effects are not obtained in regard to the integrating (I) operation and, hence, only the proportional (P) operation is realized.
Thus, most of the conventional active vibration isolators apply the manipulated variable to the support mechanism of the vibration isolation platform as damping by feeding back the output of the vibration measurement means (e.g., acceleration sensor). In other words, only the sky-hook damper is utilized. The superiority of the sky-hook damper with regard to a damping effect using a passive element is obvious. However, in order to realize the full potential of an active vibration isolator, the sky-hook spring effect should be manifested and not just the sky-hook changes effect.